1. Field of the Invention
The present invention generally relates to step-down switching regulators for supplying power to electronic devices. Particularly, the invention relates to technologies for protecting a power supply circuit and a load circuit upon termination of power supply by quickly lowering a voltage applied to an electronic circuit and preventing the development of an output voltage undershoot due to an inductor.
2. Description of the Related Art
When an operation of a power supply circuit supplying power to a load is stopped, it is necessary to quickly lower the voltage inputted to the load. This is because a delay in lowering the applied voltage may lead to an extended application of a low voltage outside a guaranteed operation range to a load electronic circuit, resulting in a malfunction of the electronic circuit or other unexpected troubles.
In high-efficiency power supply circuits used for small electronic devices, a non-insulated type switching regulator employing an inductor is widely used. In this type of power supply circuits, the use of the inductor may lead to the development of a high voltage across the inductor unless a path is provided for discharging the energy stored in the inductor after turning off of power supply. Such a high voltage often causes malfunctioning or other troubles of the electronic components of the power supply circuit or an electronic circuit connected to an output terminal.
Japanese Laid-Open Patent Application No. 2007-288863 (hereafter referred to as “Patent Document 1”) discusses a technology whereby the charge stored in an output capacitor is discharged by causing a first switching element in a switching regulator of which one end is connected to ground voltage to turn on when the load circuit is turned off.
Japanese Laid-Open Patent Application No. 2004-336972 (to be hereafter referred to as “Patent Document 2”) discusses a technology wherein a series circuit of a resistor and a transistor is connected in parallel with a synchronous rectification switching element. The transistor is turned on after turning off of power supply in order to prevent an undershoot that develops at a power-supply-side terminal of an inductor upon turning off of power supply.
In the switching regulator according to Patent Document 1, the first switching element functions as a switching element for synchronous rectification. Immediately after turning off of power supply, both the main switching element and the synchronous rectification switching element are turned off. As a result, a large negative voltage is produced at the power-supply-side terminal of the inductor immediately after turning off of power supply.
Because the aforementioned negative voltage is applied across the synchronous rectification switching element, a current flows from the ground voltage to the inductor via a parasitic diode of the synchronous rectification switching element. However, if the power supply is turned off by the activation of an overcurrent protection function or the like, the parasitic diode may be destroyed by the large energy stored in the inductor. The destruction of the parasitic diode leads to a loss of the function of the synchronous rectification switching element, resulting in a failure of the power supply circuit per se.
In the switching regulator according to Patent Document 2, the charge stored in the output capacitor is discharged via the inductor and the resistor, so that it takes time for the output voltage to decrease after turning off of power supply. Thus, there remains the problem of the extended application of a low voltage outside a guaranteed operation range to an electronic circuit connected to the output terminal.